This is meant to be nothing more than a collection of things I hope we end up seeing in future books, and some things I think would be cool but maybe not canon. I'm not trying to guess at any major plotlines, just put out some fun ideas.
Needless to say, there will be heavy spoilers for the entire series. Text wall incoming.
Time Travel. We've barely done anything with chronomancy in the series, despite how cool its implications can be. McCoy did mention that creating a paradox might or might not create alternate timelines, and Chekhov's Gun (which Jim Butcher is really good at adhering to, so far) might make that come up, and anyone who's read Jingo knows how well that can be done. More than anything else, though, I want to see a story where Harry has to go back in time to protect Baby Harry or Storm Front Harry from a time-traveling threat. Seeing his current perspective on his past self would be funny.
Siblings. I know what you're thinking: "But, Shell, Harry already has a sibling! Ortega's comment in Death Masks about that was completely fulfilled!" Yes, I know, but here's the thing: Ortega called Harry the youngest of Margaret LeFay's children. Youngest, to me at least, is a superlative (young, younger, youngest), implying at least 3 of a set. Harry and Thomas might have another brother, or maybe a sister this time. If said third sibling is the eldest, he/she might be evil, representing Margaret's much-vaunted darker nature. Having to go up against his own flesh and blood for the fate of the world isn't something Harry's been put through yet, so it's probably coming soon. Something on a little bit of a lighter, if less canon, note, would be if Harry had a sibling from his father's side. This wouldn't make a lot of sense, canonically ("Why wouldn't Malcolm have told Harry about another child?"), but it would be really fun to have a completely vanilla mortal walk up to Harry and say "I'm your brother, Harry." Harry having to protect a completely normal human from the supernatural world, or the knowledge thereof, is just an idea I like.
Arthur. We know that the Arthurian legend is mostly accurate, given the existence of Excalibur and Merlin, and that Merlin is currently trapped under Demonreach. As everyone knows, the end of the legend states that the Once and Future King will come again when Britain most needs him. An apocalypse of the kind Butcher has been hinting at might be exactly that time. I'm sure everyone here wants to see Arthur rising from Avalon (an area of the Summer Court, naturally), taking up Amoracchius, summoning his Knights (maybe the current Knights of the Cross, maybe resurrected/reincarnated versions of Gawain, Tristram, Percival, etc. Not Lancelot, he sucks.) and Merlin, and doing battle with an evil foe. We've also had at least one hint that he fought Outsiders in his time (Morgana's athame being infected with Nemesis) so that might be a thing.
Namshiel. This is something I don't want to see, but I want to know if it's true or not. In Small Favor, it was HEAVILY implied that Namshiel is infected with Nemesis. At the end of Battle Ground, Marcone now has Namshiel in his head, and I don't know whether or not that would mean he gets infected, too. If it does, that would be bad. Marcone as a villain would be bloody terrifying.
Bonea. At this point, I have to say it. What the heck, Jim? Bonea is just as much Harry's daughter as Maggie is, but we've only gotten like two sentences of interaction between them. I must see more, and more than that, the two of them deserve more time together. Bonea has the potential to be really interesting (a completely innocent spirit of intellect with strong ties to Harry and maybe a soul), but not if she gets absolutely no screentime.
Puck. All right, this one's a bit more out on a limb, but bear with me. For anyone out there who's read Shakespeare, A Midsummer Night's Dream, to be exact, you'll know that that's where a lot of the notions about the Summer Court of the Fae comes from. Butcher has stayed pretty close to those ideas (although he did cut out Oberon, which makes me sad), but so far we haven't seen one of the most integral characters to the story: Puck. Puck is the Jester of the Summer Court. He loves mischief and laughter, and having him show up to plague Harry because Titania is still mad about the whole thing with Aurora would be a very... Fae thing to do. "Oh, but I just sent my Jester to amuse him, as a sign of respect for his abilities. How was I supposed to know that Puck's innocent little jokes would send your Knight to his death?" One last note on that thought, without any evidence to support it whatsoever, neither Court can have a position that the other doesn't. No imbalance is allowed. So, if Summer has a Jester, Winter must also- no matter how hard to job of making Mab laugh is. Hear me out: Jack Frost. That is all I will say on the matter, except that it might end up being one of his duties to amuse the Winter Knight.
Other Mythologies. This is just a general note about the Mythos we've seen so far. There's been a lot of Norse and Greek myths come to life, as well as many Celtic beings. We've seen hints that most, if not all, legends are true in some way, so seeing more of them come to life would be epic. I, for one, would love to see Cu Chulainn ripping through legions of monsters or Momotaro cutting down Oni.
Merlin. This was probably covered by the bit I wrote about Arthur before, but I really want to know more about the history of the Council and of Demonreach, and how Merlin fits in with everything that's ever happened. I know, I don't ask for much, but you can't tell me you're not also curious.
DuMorne. Here's another limb-jump. Throughout the series, every single time DuMorne has come up, Butcher has made a specific point to remind us of how dead he is. (Talking about them finding his body, people ascertaining he died, etc.) I honestly can't tell in this situation, though, if methinks he doth protest too much is accurate. Butcher could be making this much of a point to make it that much more shocking when DuMorne makes a triumphant return, or I could just be reading way too far into an attempt to clarify that he isn't actually coming back. Thoughts?
The Black Court. In Battle Ground, Wild Bill, Yuki, and probably Chandler got killed, and thus most likely turned by members of the Black Court. If they come back, as extremely powerful wizard-vampires, it will certainly fall to Harry and Ramirez to deal with them, in what will surely be a heart-wrenching scene that I'm simultaneously dreading and looking forward to.
The Jade Court. Maybe possibly unless?
Ferrovax. I'm actually not going where you think on this one. I could easily go on a 20-page rant about how cool dragons are and how Ferrovax deserves more time in the spotlight, but instead I'll just ask one question: At Bianca's party, Lea received a gift infected with Nemesis. Ferrovax also received a gift. What was it?
All right, I think that's it. I haven't included anything that's definitely, absolutely going to happen (like more fights with Outsiders or a romance between Harry and Lara), instead trying to focus on possibilities that seemed interesting. If you have any thoughts or questions about these, or any cool ideas of your own, please comment down below! I look forward to hearing from you.